In recent years, there is an increasing demand for small and lightweight secondary batteries having a high energy density along with the spread of portable devices such as cellular phones and laptop personal computers. As such a secondary battery, there is a nonaqueous electrolytic solution type lithium ion secondary battery, the research and development thereof has been actively carried out, and it has been attempted to put the battery to practical use. Such a lithium ion secondary battery includes a positive electrode containing a lithium-containing complex oxide as an active material, a negative electrode containing a material capable of occluding and releasing lithium such as lithium, a lithium alloy, a metal oxide, or carbon as an active material, and a separator containing a nonaqueous electrolytic solution or a solid electrolyte as main constituents.
Examples of the material to be investigated as a positive electrode active material among these main constituents may include lithium-cobalt complex oxide (LiCoO2), lithium-nickel complex oxide (LiNiO2), and lithium-manganese complex oxide (LiMn2O4). In particular, a great number of batteries using lithium-cobalt complex oxide in the positive electrode have been so far developed to obtain excellent initial capacity characteristics and cycle characteristics, various results have already been obtained, and the batteries have been put to practical use.
Meanwhile, recently, the application fields of lithium-ion secondary batteries include a number of different fields from portable devices to vehicle applications such as hybrid vehicles and electric vehicles, and the characteristics required to secondary batteries have changed. In particular, a higher energy density of the battery is required in the vehicle applications described above. Hence, it has been investigated to use a positive electrode active material having a high potential, and among these, lithium-manganese-nickel complex oxide which stably expresses a high voltage of about 4.75 V in terms of metal lithium potential has attracted attention.
However, lithium-manganese-nickel complex oxide has not yet been put to practical use as a positive electrode active material at the present time. One reason for this is that lithium-manganese-nickel complex oxide has a high resistance. In addition, a highly resistive nonconductor film is formed on the surface of lithium-manganese-nickel complex oxide, and this leads to an increase in internal resistance and deterioration in battery characteristics, and further, it is also concerned that a decrease in safety is caused.
One of the factors of the formation of nonconductor film is the presence of impurities contained in manganese-nickel complex hydroxide which is a starting material for lithium-manganese-nickel complex oxide. In other words, when preparing lithium-manganese-nickel complex oxide by calcining manganese-nickel complex hydroxide and a lithium compound of a lithium source, a sulfate salt such as electrically inactive lithium sulfate and the like are formed on the surface layer of the positive electrode active material as a nonconductor film as the sulfate radical (SO4) and the like of impurities in the manganese-nickel complex hydroxide react with the lithium compound, and an increase in internal resistance and deterioration in battery characteristics are thus caused.
In order to cope with such a problem, for example, Patent Document 1 proposes a technique to decrease the sulfate radical by washing the synthesized lithium transition metal complex oxide with water. However, in such a method, the washing step is further required and the cost increases as the steps increase. In addition, it is concerned that the battery capacity decreases since lithium is eluted together with the sulfate radical at the time of water washing.
In addition, Patent Document 2 discloses a method for suppressing mixing of the sulfate radical into the oxide by using a manganese compound having a small content of sulfate radical as a starting material at the time of the synthesis of lithium-manganese oxide. However, it is concerned that the kiln is damaged by chlorine generated at the time of the calcination and synthesis, for example, when a chloride salt, which is a representative example of a metal salt other than a sulfate salt, is used. In addition, sulfate salts such as manganese sulfate are inexpensive as compared to salts of other anions and it is thus disadvantageous not to use a sulfate salt as a starting material from the viewpoint of cost.
Meanwhile, Patent Document 3 discloses the use of manganese-nickel complex hydroxide having a molar ratio of manganese:nickel of 1:1 for the characteristic improvement of battery performance. In this patent document 3, the average particle diameter of the particles, the specific surface area thereof, the sulfate radical contained therein, and the peak intensity ratio obtained from the X-ray diffraction spectrum thereof are described. However, the particle size distribution of the particles is not described. The expanse of particle size distribution of the hydroxide particles of a starting material is a significantly important characteristic when producing lithium-manganese-nickel complex oxide from a complex hydroxide, and specifically, it affects the occurrence of abnormal aggregation and the like of lithium-manganese-nickel complex oxide to be obtained. Moreover, such abnormal aggregation leads to an increase in resistance of the battery formed to contain the lithium-manganese-nickel complex oxide as a positive electrode active material.
As described above, the expanse of particle size distribution of the hydroxide particles of a starting material is a significantly important characteristic in the production of lithium-manganese-nickel complex oxide, and the method of Patent Document 3 alone is not sufficient in order to achieve a secondary battery having higher performance.
As described above, manganese-nickel complex hydroxide having a small content of impurities and controlled particle size distribution, and lithium-manganese-nickel complex oxide obtained by using the manganese-nickel complex hydroxide as a starting material have been desired.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2001-273898
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2000-306577
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2004-210560